In conventional pressure diffusers, for washing or otherwise treating with treatment liquid cellulosic (paper) pulp, such as sold by Kamyr, Inc. of Glens Falls, N.Y. under the trademark MC, or in other pressure diffusers, such as those shown in U.S. Pat. Nos. 4,944,167 and 5,187,956, a screen assembly is moved slowly (at approximately the same speed of movement as the pulp) in the direction of pulp flow until it reaches the end of its stroke in a first direction, and then it is moved rapidly (i.e. at least four times more quickly than in the first direction, typically more than ten times more quickly) in a second, opposite, direction in order to cause [in conjunction with the conical geometry of the screen assembly] backflushing of the perforated screen outer surface of the screen assembly to prevent it from being clogged. In order to obtain optimum washing efficiency it is intended that the liquid backflushing the pulp at any particular area be liquid withdrawn from the pulp at that same area. Though conventional pressure diffusers exhibit excellent washing efficiency, the distribution of the backflush can be further optimized such that pressure diffuser washing efficiency and ease of operation can be improved further.
In conventional pressure diffusers, during rapid backstroking of the screen assembly (typically downstroking), pulp adjacent the pulp inlet is compressed by screen assembly components. This compression prevents backflushing liquid at the compressed area so from flowing through the screen into the pulp, and instead causes this liquid--which is typically the "dirtiest" liquid in the pressure diffuser--to flow toward the pulp outlet until it moves past the compressed area, and then into relatively clean pulp thereat. This compression also restricts the flow of clean treatment (typically wash) liquid into the annulus between the interior surface of the pressure vessel and the exterior surface of the screen; the liquid is typically introduced through "wash baffles." This causes restrictions, channeling, and reduced flow into the compressed area, causing undesired thickening adjacent the pulp inlet, and causes more wash liquid than desired to flow into other areas of the pressure vessel, causing undesired dilution.
The invention alleviates both of the above-described drawbacks. According to the first aspect of the invention, the flow of fresh wash liquid is normalized during rapid backstroking of the screen assembly by providing separate headers and flow controls for supplying wash liquid to the compressed pulp area and other areas of the vessel. According to a second aspect of the invention, flow of dirty backflushing liquid to cleaner volumes of pulp is restricted, and instead, the backflushing liquid is caused to perform its desired screen-backflushing function at the compressed pulp area (from which it was originally removed). While the aspects of the invention may be used individually, they are preferably used together, and complement each other and optimize the efficiency of the system. Both aspects may be readily retrofit to existing installations, or incorporated in new installations, and are simple.
According to the first aspect of the invention, a pressure diffuser assembly is provided comprising the following elements. A superatmospheric pressure vessel elongated in a dimension of elongation, and having an inner wall, an inlet and an outlet for pulp, so an outlet for withdrawn liquid, and a plurality of inlets for treatment liquid along the dimension of elongation thereof. A screen assembly mounted within the vessel and defining a pulp flow annulus between an exterior surface thereof and the inner vessel wall, pulp flowing from the inlet to the outlet through the annulus generally in a first direction along the dimension of elongation, and defining an interior withdrawn liquid volume communicating with the outlet for withdrawn liquid. Means for moving the screen assembly for slow movement in the first direction along the dimension of elongation, and for rapid, backflushing, movement in a second direction, opposite the first direction, rapid movement in the second direction causing pulp compression adjacent the pulp inlet. And means for providing continued substantially constant volume flow of treatment liquid through the inlets for treatment liquid, including treatment liquid inlets adjacent the pulp inlet, despite compression of the pulp caused by rapid movement of the screen assembly in the second direction.
Preferably, a plurality of sets of horizontal treatment liquid conduits are connected to the inlets along the dimension of elongation, and at least one vertical header is connected to the conduits. In this case, the means for providing continued substantially constant volume flow of treatment liquid through the inlets comprises a first vertical header and flow control assembly for one or more sets of horizontal treatment liquid conduits adjacent the pulp inlet, and at least a second vertical header and flow control assembly for one or more horizontal treatment liquid conduits remote from the pulp inlet. The vertical headers (e.g., first and second headers) may comprise a common pipe with an isolation device precluding flow between them, or the vertical headers may comprise distinct pipes supplied with treatment liquid by a common pump.
Each of the flow control assemblies preferably comprises a flow so element operatively connected to a flow control valve. Each of the headers preferably comprises at least two branches spaced circumferentially around the superatmospheric pressure vessel, and a common pump supplies treatment liquid to the first and second vertical headers. The headers may be fed by individual pumps.
According to the first aspect of the invention, a method of operating a pressure diffuser assembly for treating cellulosic pulp with a treatment liquid is provided. The assembly includes: a superatmospheric pressure vessel elongated in a dimension of elongation, and having an inner wall, an inlet and an outlet for pulp, an outlet for withdrawn liquid, and a plurality of inlets for treatment liquid along the dimension of elongation thereof; a screen assembly mounted within the vessel and defining a pulp flow annulus between an exterior surface thereof and the inner vessel wall, pulp flowing from the inlet to the outlet through the annulus generally in a first direction along the dimension of elongation, and defining an interior withdrawn liquid volume communicating with the outlet for withdrawn liquid; and means for moving the screen assembly for slow movement in the first direction along the dimension of elongation, and for rapid, backflushing, movement in a second direction, opposite the first direction, rapid movement in the second direction causing pulp compression adjacent the pulp inlet. The method comprises the step of (a) providing continued substantially constant volume flow of treatment liquid through the inlets for treatment liquid, including treatment liquid inlets adjacent the pulp inlet, despite compression of the pulp caused by rapid movement of the screen assembly in the second direction.
Step (a) is typically practiced by supplying treatment liquid to both the first and second vertical headers, separately controlled by the flow control assemblies associated therewith. There is also preferably the further step (b) of distributing liquid from a common pump to the flow control assemblies associated with the first and second vertical headers, steps (a) and (b) preventing undesired thickening of pulp adjacent the pulp inlet.
According to the second aspect of the present invention, a pressure diffuser assembly is provided comprising the following components: A superatmospheric pressure vessel elongated in a dimension of elongation, and having an inner wall, an inlet and an outlet for pulp, an outlet for withdrawn liquid, and a plurality of inlets for treatment liquid along the dimension of elongation thereof. A screen assembly mounted within the vessel and defining a pulp flow annulus between an exterior surface thereof and the inner vessel wall, pulp flowing from the inlet to the outlet through the annulus generally in a first direction along the dimension of elongation, and defining an interior withdrawn liquid volume communicating with the outlet for withdrawn liquid. The screen assembly includes an outer perforated screen and a concentric inner screen support cylinder radially spaced from the outer perforated screen. Means for moving the screen assembly for slow movement in the first direction along the dimension of elongation, and for rapid, backflushing, movement in a second direction, opposite the first direction, rapid movement in the second direction causing pulp compression in a pulp compression area adjacent the pulp inlet and tending to cause backflushing of withdrawn liquid from the screen assembly adjacent the pulp inlet through a portion of the screen assembly past the pulp compression area in the first direction. And one or more rings mounted between the outer perforated screen and the concentric inner screen support cylinder and having a plurality of rapid-flow restricting openings therein. A ring with flow restricting openings substantially prevents backflushing liquid from moving from the pulp compression area to areas past the pulp compression area in the first direction.
A ring preferably comprises a plurality of generally semi-circular segments mounted together in substantially liquid tight relationship. A ring has an outside diameter substantially the same as or slightly less than the inside diameter of the outer perforated screen, and an inside diameter substantially the same as or slightly more than the outside diameter of the inner support cylinder, so that the ring is held in liquid tight engagement between the outer perforated screen and the inner support cylinder.
The rapid-flow restricting openings in a ring may comprise fixed or adjustable: rounded surface orifices, bevel-edged orifices, sharp-edged orifices, slots, openings with check valves allowing flow only in the second direction and one or more adjacent bleed orifices, or combinations thereof. The ring may have between about 8-90 rapid-flow restricting openings and a thickness of about 0.125-2 inches. Where at least some of the rapid-flow restricting openings comprise fixed rounded surface orifices, they preferably have a first end with a diameter of about 0.5-8 (preferably 2-4) inches and a second end, downstream of the first end in the second direction, of about 0.5-3 (preferably 0.5-1.5) inches with a smooth and curved wall therebetween.
It is the primary object of the present invention to provide a pressure diffuser assembly, and method of operation, optimizing washing (treatment) efficiency in a simple yet effective manner, which can be employed in existing installations and new constructions. This and other objects will become clear from an inspection of the detailed description of the invention, and from the appended claims.